The Global Positioning System (GPS) is a satellite-based system developed by the United States Department of Defense to provide accurate positional information to a GPS receiver anywhere on earth, including its altitude. The GPS system comprises a constellation of 24 or more earth orbiting satellites in 12-hour orbits. The satellites are arranged in six orbital planes, each containing four satellites, spaced sixty degrees apart and inclined approximately fifty-five degrees from the equatorial plane. The constellation configuration ensures that between four and twelve satellites are visible at any time at any location on earth with a clear view of the sky. A properly equipped GPS receiver typically yields positional coordinates in three dimensions.
The GPS satellite signal includes a carrier signal that is bi-phase modulated with a 1023 bit long Gold spreading code at a 1.023 MHz chip rate, corresponding to a 0.001 second repeat interval. The carrier signal also includes a navigation data message modulated at 50 bits per second (BPS) (transmitted at a rate of twenty milliseconds per data bit). The navigation data message includes GPS satellite data, including satellite position (ephemeris) data and satellite clock correction data, for use by GPS receivers, and information for determining GPS time (i.e., a clock time of the GPS satellite) and information for determining geographical location.
Precise time is required for accurate GPS positioning. An accurate estimate of satellite position associated with several corresponding pseudorange measurements is required to derive an accurate position estimate, and knowledge of the signal transit time between the GPS receiver and each satellite is required to determine the pseudorange measurements. In order to predict satellite positions on the order of one meter, for example, time must be acquired to an accuracy of 1 millisecond, since peak range rates attributable to satellite motion are roughly 1 km/sec. Without accurate time information, GPS receiver code phase measurements are ambiguous since they represent only a fractional portion of the pseudorange measurements within 1 millisecond, which is the period of the GPS commercial pseudo random code.
The clock time included in the GPS message is an absolute time signal that is precisely synchronized at the satellite. Particularly, all satellites in the GPS constellation are synchronized by ground reference stations, which precisely control the absolute time error of the satellites on the order of several nanoseconds.
The absolute GPS time signal may be used by GPS receivers to accurately determine time and position. Once position is known approximately, the absolute time in the GPS receiver can be determined by offsetting the precise time observed by the GPS receiver in the satellite broadcast message by the computable propagation delay between the receiver and the satellite. It is thus desirable for GPS receivers to be able to obtain accurate time measurements.
GPS receivers cannot always reliably determine a local time from the GPS satellite broadcast navigation data message. Due to their portable nature, such GPS receivers, for example, those incorporated in cellular telephones and hand-held devices, are often used in vehicles traveling in urban canyons and in buildings and in other environments that obstruct or significantly degrade the satellite signals. In these environments, reception of the 50 BPS absolute time signal of the navigation data message is unreliable.
A Time Of Week (TOW) data field included in the 50 BPS navigation data message, in conjunction with the absolute time signal, allows GPS receivers to accurately and reliably determine local time. The TOW data is transmitted by all satellites at six-second intervals. Detection of the TOW data is dependent on signal magnitude. Below a certain signal magnitude level it is possible to obtain a range measurement, but it is not possible to decode the TOW data. For example, at signal levels below approximately 30 dB-Hz, it is nearly impossible to decode individual message bits of the 50 BPS message. It is possible, however, to obtain signal correlation at signals substantially below 30 dB-Hz, perhaps below 20 dB-Hz. Not only is it desirable to obtain accurate time in GPS receivers, it is also desirable to determine time accurately in weak signal environments, for example at signal levels below 30 dB-Hz.
U.S. Pat. No. 5,945,944 entitled “Method And Apparatus For Determining Time For GPS Receivers” discloses the establishment of GPS time in a wireless communications handset by demodulation of signals received from cellular communications infrastructure.
U.S. Pat. No. 5,812,087 entitled “Method And Apparatus For Satellite Positioning System Based Time Measurement” discloses determining GPS time by comparing portions of satellite navigation data messages overlapping in time, but this scheme is usable only where time is in error by at most a few seconds.
U.S. Pat. No. 6,346,911 entitled “Method and Apparatus For Determining Time in a GPS Receiver” discloses a method of measuring the time of arrival of a predicted sequence of bits by a data correlation method, the time associated with the predicted sequence of bits being known.
The various aspects, features and advantages of the present invention will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description of the Invention with the accompanying drawings described below.